For example, load analyses of tiered storage systems including hard disk drives (HDDs) and solid state drives (SSDs) have revealed that nomadic work load spikes emerge in some of the tiered storage systems.
As used herein, the term “spike” refers to the situation where work loads (also known as loads) emerge intensively on a limited area in a storage. The term “nomadic work load spike” refers to a situation where such spikes occurs intensively for a relatively shorter time (e.g., about one to 10 minutes), and then spikes emerge in a different location (offset).
An example of nomadic work load spikes is illustrated in FIG. 13.
In FIG. 13, the horizontal axis represents offsets in the volume in a storage, while the vertical axis represents elapsed time.
The black bars in FIG. 13 represent 1-GB segments where work loads inventively emerge in the storage volume. Here, the term “segment” refers to fixed-size area in the storage volume. For example, a 1-GB segment is 1-GB area in the volume in the storage.
At the time of the arrow A in FIG. 13, ten or less segments are hit by spikes simultaneously. The arrows B and C in FIG. 13 indicate segments where work loads last.
For eliminating such work loads, in addition to HDDs, tiered storage systems are provided with an SSD as a cache, for achieving both performance improvement and cost efficiency. The scheme where an SSD is employed as a cache is referred to as the SSD cache scheme.
Examples of SSD caches include Facebook flush caches and Fusion Direct caches.
Unfortunately, such an SSD cache scheme employs the writeback of the cache. Hence, the SSD cache scheme may cause a problem upon migrating nomadic work load spikes with higher write ratios.
Specifically, once all SSD cache blocks have been consumed, for allocating a new spike and cache block, some cache blocks need to be cleaned. For nomadic work load spikes with higher write ratios, a significant amount of writeback to HDDs occurs.
Typical SSD cache blocks have smaller sizes, e.g., 4 kilobytes (KB), and hence a writeback causes a random access to the HDD, which leads to a significant delay.
Additionally, in the writeback cache scheme, once all cache areas are exhausted, a writeback of a dirty block (a block in the SSD the content, data in which does not match the content in the corresponding block in the HDD) occurs frequently. While nomadic work load spikes with higher write ratios are executed, writeback of dirty blocks frequently occurs, which consumes significant areas that can be used by the user.
For the reasons set forth above, applying an cache SDD to work loads experiencing nomadic work load spikes with higher write ratios is often not so effective as expected.